The present invention relates to a coating for a ski for variable snow conditions. More particularly, the invention relates to a coating for a ski to be used in cross-country or long distance skiing and to a method of manufacturing same.
The invention further relates to a ski for variable snow conditions, in particular to a cross-country or long distance ski, of which the bottom gliding surfaces are coated with polyethylene or another material having a low enough coefficient of gliding friction and where the central area of the sliding bottom coming into contact with the snow during the push-off phase, coated with a material having elastic behavior, is formed to constitute a holding bottom, or adhesion coating.
The proper functioning of a ski intended for cross-country or long distance skiing, that is, the adhesion and gliding functions, depends on the fact that the adhesive friction, or the friction at rest between the ski's bottom surface and the snow, is high and the gliding friction, or the friction of motion, is low. If this condition is satisfied, it is possible to push off well and to glide well with the ski. On the other hand, the snow contributes substantially to the creation of favorable friction properties with regard to the holding and gliding capacity of the ski. The condition of snow changes with changes of temperature and with the age of the snow crystal. It has therefore been necessary, as is well-known, to wax cross-country or long distance skis. One had to spread over the bottom surface of the ski a layer of a certain wax-like substance which was appropriate to the purpose of obtaining desired friction characteristics for the snow existing and in the conditions prevailing in each instance.
The adhesive friction or the friction at rest between snow and ski, that is the holding capacity, is dependent in the first place on the crystal form of the snow. It is known that the crystal form of snow changes in dependence upon the age of the snow crystal, the snow temperature and the moisture content. The snow crystal has numerous different forms, but essentially it is in this connection the sharpness of the crystal points. New snow that has fallen in sub-zero weather has very sharp points and a fine distribution. With increasing age of the snow, the crystals grow together, whereby their points are blunted.
When the temperature rises close to zero degrees C, the moisture content of the snow increases, and simultaneously the sharp points of the crystals melt. Due to the influence of the moisture, the inner friction of the snow increases and the crystals adhere easily to each other, and also to surfaces which absorb the water.
It is possible in the case of frost snow, which has sharp points, to achieve sufficient adhesive friction with an elastically behaving material with suitable hardness, into which the sharp points of the snow crystals can enter. Known materials of this kind are, for example, polyurethane elastomer, rubber, or the equivalent. The functioning range is favorably widened by selection of an elastomer with a hardness which is as strongly temperature-dependent as possible.
On transition to temperatures above the freezing point, the pre-conditions for creation of adhesive friction change, because the water present on the surface of the snow crystals forms a film on the ski's running surface. This influence prevents direct contact of the crystals with the bottom material; the water film acting as a friction-reducing lubricant in this connection.
It is possible by successful waxing to achieve both good gliding and good holding capacity of the ski. However, selection of appropriate waxes for different snow conditions and the waxing process require profound professional skill and experience, and it is a fact that in many instances even specialists fail in the art of waxing.
It is possible, on the other hand, for snow conditions to change very rapidly, so that the adhesion and/or gliding capacity may, for example, be substantially impaired during one competition. It is also a drawback of waxed ski running surfaces that the waxing agents are consumed very rapidly in certain circumstances.
It would thus be a decisive advance if a ski bottom surface or a ski coating could be created which possesses good gliding properties as well as good holding or adhesion properties in varying snow conditions.
As known, the function of ski waxes is based on the circumstance that the sharp points of the snow crystals penetrate into the elastic or plastic superficial layer formed by the waxing agents and thereby produce a high enough friction at rest or adhesive friction to provide the ski with its holding capacity. The gliding of the ski occurs, in the case of waxed as well as unwaxed skis, upon a thin water film due to the fact that the points of the snow crystals melt under the influence of the friction of motion.
Various ski bottom or ski coating embodiments known in the art consist of active and passive parts, which penetrate into the snow and produce a mechanical holding action. In this regard, a ski coating known in the art has a surface profiled in fish-scale fashion. The scales form inclined planes along which the ski glides. The rearwardly oriented vertical steps of the scales prevent backward gliding and produce holding properties due to the banking up of the snow in front of them.
A recessed and stepped bottom or coating is also known in the art. The functioning of such bottom is based on the same mechanism as that of the scaled coating, with a negative profile, however. This bottom displays better gliding behavior, whereas its holding or adhesion capacity is poorer than that of the scaled coating. Norwegian publicizing print No. 89238 is an example of this solution to the problem.
Various hairy ski bottoms or coatings are also known in the art. This coating, such as, for example, synthetic hair strips, is applied to the bottom. The hairs are disposed with an inclination in the direction of gliding. In these bottoms, movement directed against the hairs causes the holding or adhesion of the ski in the push-off process. German publicizing print No. 1 144 165 and Finnish Pat. No. 43401 are examples of these known bottoms.
Also known in the art are ski bottoms or coatings or running surfaces of slick plastics, for example, Teflon, having a characteristically minimal friction coefficient. U.S. Pat. No. 2,908,506 is an example of such bottoms. In these skis, however, the holding problem is not solved.
Furthermore, various ski embodiments are known in the art in which an effort has been made to improve the inherently poor adhesion capacity towards wax agents which many plastic coatings display. An example of this is disclosed in U.S. Pat. No. 3,897,074, which is equivalent to Finnish Pat. No. 43401. As disclosed in these patents, it is known in the art to carry out the surface treatment of the bottom or running surface by grinding to produce a nap-like surface on the ski bottom. This provides the gliding capacity of the ski.
It is typical of all the previously known mechanical ski bottom structures hereinbefore mentioned that they function relatively well within a relatively narrow range of limited snow conditions, but that they are nearly unusable in conditions other than those of the narrow range. Thus, for example, the scaled bottom operates very well on both wet snow and soft new snow, but it is unable to satisfactorily prevent the backward slipping of the ski on a glazed sub-base. The functioning of the scaled bottom is further impaired by its rapid wear, since the holding of the surface of the ski bottom is only effective if the rear edges of the scales are sharp. The other embodiments hereinbefore discussed also have certain drawbacks in each case. Thus, they have not become popular with skiers.
Regarding the state of art, particular reference is made to Finnish Pat. No. 43401 of the same applicant. As therein disclosed, a textile fabric is used for the ski bottom material. The fibers of the fabric extend at least up to the boundary surface of the gliding surface of the ski bottom. In this solution of the problem, the principal object is to achieve good adhesion of the ski wax compounds to the gliding bottom via the fibers. However, certain drawbacks are also apparent in this gliding bottom. One drawback is that the manufacture of the gliding bottom is difficult. Another drawback is that the fibers wear down and break rapidly.